1. Field of the Invention
The invention relates to driven, headed, and screw-threaded fastenings. More specifically, the invention relates to an expansion-type bolt suited for anchoring in rock, as in mines and the like.
2. Description of the Prior Art
Various types of expansion bolts have in the past been used for mining and other applications. The uniform object of such known expansion bolts is to create a structure that will engage the surrounding rock of a predrilled hole and support a load applied to the bolt.
In the mining industry, one of the primary functions of such expansion bolts is to hold the roof of the mine in place against collapse. For this purpose, expansion bolts known as roof bolts have been employed. Prior to the use of roof bolts, mine roofs were supported by timbers in post and lintel construction. The lintels carried the weight of the mine roof. With the greater use of mobile machinery in mining, the danger arose that the post timbers might be bumped by the machinery, resulting in the collapse of a post and lintel, and further resulting in the collapse of a portion of the mine roof.
The roof bolt eliminated the need for post and lintel construction. It was discovered that the mine roof could be supported by bolts having their anchor point at a predetermined distance above the mine roof. Hence, it became the modern practice to bore long holes in the mine roof, for example, seven feet long, and anchor a seven foot long expansion bolt in the hole, with the expansion portion at the furthest end of the hole.
Properly setting a roof bolt in the rock is preferably accomplished by applying a known torque to the bolt. Since the roof bolt is expected to support a predetermined weight of rock against collapse, the consequences of improperly installing the bolt are severe. It is also desirable to assure that the bolt is engaging the walls of the predrilled hole in which it is anchored with uniform force throughout the circumference of the expansion portion, providing maximum protection against slippage. These functions are accomplished by proper design of the expansion portion and proper tightening of the bolt.
In the prior art, expansion bolts are known to employ a split sleeve over the bolt end, and this sleeve is cammed outwardly with the split providing the region at which the portions of the sleeve pivot away from one another. A disadvantage of this construction is that the expansion portion does not necessarily apply pressure evenly in circumferential distribution, and some portion of the split sleeve may be overly deformed during tightening of the bolt. Any resultant slippage of the bolt can lead to a hazardous condition in the mine.
A further problem in the prior art is that a roof bolt is required to perform two functions that may interfere with one another. First, the bolt should firmly engage the deepest part of a predrilled hole with radial pressure. Second, the bolt should apply slight pressure between the deeply anchored portion and the head of the bolt, placing the rock in compression and correspondingly placing the bolt in longitudinal tension. These two functions may conflict if, for example, the radial pressure at the expansion portion of the bolt locks the bolt in a longitudinal position wherein there is insufficient longitudinal tension in the bolt. This condition would result if the head of the bolt were not drawn snugly against the rock shoulder around the predrilled bolt hole before the bolt became anchored.
The present invention resolves the above noted problems in roof bolts and offers a general improvement in the construction and reliability of such devices.